The yin and yang of evolution. Several key dualities in evolution are better understood when they are viewed as complements to each other under the framework of the classic symbol of yin and yang. Common descent is the prerequisite for HGT, but horizontal innovation shapes the pattern of descent. Inheritance of genetic material is often web-like, but membrane heredity is tree-like. Both polarities of each of these dualities exist because of the other. The existence of darkness does not invalidate the existence of light, just as the prevalence of HGT does not invalidate the TOC.